The present invention relates generally to a method and a device for wrapping a plurality of soft elements, and specifically for wrapping boards of fibrous insulating material.
As a rule, equipment for wrapping insulating material is connected directly to a production line where the material is sawn and/or cut in specified sizes. This is a continuous process where the boards are usually supplied at a rate of one board per second or faster.
When wrapping pads or boards of a soft material, such as insulating material, a method has since long been applied, in which rectangular boards are stacked into a stack, compressed in the direction of stacking into a package, and then provided with a coating of a thin flexible material, such as plastic, which maintains the package in its compressed state.
According to a frequently used method, the packages are inserted in their longitudinal direction into an opening of a sack or tube of plastic, which is then possibly welded together at its open end.
To automate the handling and eliminate the need for a sack or tube, an alternative method is used, in which each package, while being continuously fed, is coated with a surrounding, but not sealed plastic material, which is first welded along the long sides of the package. Subsequently welding and cutting between the packages are carried out. By welding is here meant all kinds of joining where at least part of the wrapping material is heated during joining.
The welding along the long sides of the package is difficult to accomplish in a satisfactory manner, and complicated devices are necessary to weld the packages in a quick and reliable manner. In particular, it is difficult to apply in a satisfactory manner the necessary pressure to the joint without it being weakened, especially if the surrounding material is a plastic material.
In certain embodiments of the above method, the sides of the packages are, perpendicular to the direction of compression, subjected to a mechanical pressure, which results in an unfavourable pressure gradient. There is a risk that the fibres of the material, if any, will be damaged if excessive mechanical pressure is applied to the package. This is a major problem since most insulating materials consist of fibre materials which are sensitive to pressure being applied transversely of the fibres.
Furthermore, the compressed package, while being fed, frequently passes irregularities in the conveying paths, such as bumps or gaps. These irregularities are propagated through a plurality of boards in the compressed package and result in additional pressure gradients and damage the fibres.
A first object of the present invention is to provide automatic wrapping, which in every respect is satisfactory, of an insulating material, without the material being subjected to detrimental pressure gradients.
A second object of the present invention is to provide a simple and strong seal of packages of compressed insulating material.
These objects are achieved by a method which comprises the steps of stacking the elements, compressing the stack in a direction of stacking, thereby forming a package, inserting the package in a feeding direction into a film, preferably a plastic film, which at least partly consists of a thermoplastic material and at least in a partly overlapping manner surrounds the package, sealing the film in the feeding direction, and cutting and optionally sealing the film transversely of the feeding direction. The method is characterised in that the sealing of the film in the feeding direction is carried out by applying heat to the portions of the film which overlap each other so that they change into an at least partly molten state, and by supplying, from a nozzle means in the direction of the film, a pressurised fluid so that the portions are joined.
The expression film relates to a thin and flexible material, which is capable of tightly surrounding the compressed material.
The method can be accomplished by means of a device according to independent claim 8.
By the compression of the overlapping portions occurring by means of pressurised fluid, mechanical action is avoided and the risk of weakening the joint is eliminated. In a preferred embodiment of the invention, a nozzle supplies a heated, pressurised gas, preferably air, in the direction of the overlapping portions of the film. In this manner, the gas flow causes both heating and compression of the film.
To prevent overheating of the joint, which may result in reduced strength and holes in the film, the heating is interrupted preferably in connection with an interruption of the feeding of the package. If the heating occurs by means of heated gas, the interruption may take place by the gas flow being diverted through an outlet of the nozzle means. The diverted flow of air then produces a negative pressure outside the nozzle means, so that the flow through the nozzle means is reversed. Thus, surrounding, cooler air is made to flow past the heated joint and cool it.
Downstream of the compression of the heated portions, these can be cooled, for example by supplying cold air.
According to a preferred embodiment of the invention, a negative pressure is produced between the film and the package. This negative pressure contributes to holding the film in place during sealing. Downstream of the sealing, the negative pressure in the partly wrapped package makes the film fit close to the package and counteract expansion of the insulating material in the wrapping process. This implies that the package can be transported past irregularities in the conveying paths without the fibres being damaged.
According to an embodiment, the negative pressure is produced with the aid of a suction means on each side of the feeding path, said suction means preferably being a double-walled metal sheet which extends along a distance of the package in the feeding direction, said sheet being formed with recesses towards the package and connected to a source of negative pressure. Such a sheet can be fixedly arranged at its upstream end and, downstream together with the package, be wrapped by the film. At the downstream end of the metal sheets, where the sealing of the film has at least begun, a connection between the metal sheet and the surroundings would therefore be impossible, which means that the metal sheet will be slightly yielding in the lateral direction. A package which is fed past the suction means may thus be allowed to vary in width to some extent without causing too much resistance.
It is preferable, downstream of the sealing along the feeding direction according to a technique which is known per se, to carry out sealing and cutting transversely of the feeding direction. When a package is sealed at its downstream end, it is thus wrapped by film on all sides except one, and therefore the negative pressure from the suction means gives a still greater joining effect.
Each sealing operation transversely of the feeding direction occurs in this way between two packages which are located in a kind of tube, in which a relatively high negative pressure prevails. This results in a further advantage as the fed packages in level with the transverse sealing along a distance have no contact with the feeding assembly. Since the packages are firmly compressed, the absence of contact between continuous surfaces in this joint normally results in the material in the packages being allowed to expand, with the ensuing great pressure gradients. However, according to the described embodiment of the invention, the negative pressure in the tube which is fed over the joint contributes to resisting the expansion pressure, so that the material is not allowed to expand significantly. Pressure gradients which are detrimental to the material will thus be avoided.